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Spring-piston exhausted hybrid chamber

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: Hotwired » Tue Jul 13, 2010 8:39 pm

Look at stock springs and find one suitable for you.

e.g.

Force on piston from gas pressure = 300N

On the other side we have a totally made up spring with:

Spring Free Length = 50cm
Spring Solid Length (coils are touching, cannot compress any more) = 10cm
Spring Rate = 10N/cm

When the spring is compressed to 20cm which is a reduction in length of 30cm it is producing a force of 300N and will equal the 300N force from the gas pressure on the other side of the piston.


You'd want a spring that when compressed has a force below/equal to that of the mix you're using, enough force to push the piston back through the chamber when extended and it has to be long enough to do so (can use multiple springs).

Also, right internal/outer diameter to work within pipe/over rod that you're using.
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Unread postAuthor: ramses » Wed Jul 14, 2010 11:22 am

Note that the force from gas pressure should be calculated from pre-ignition pressure. The area behind the piston (not in the chamber) must be vented to atmosphere to avoid compressing it or drawing a partial vacuum in it.

The spring probably doesn't need to be that strong, having it fully compressed at a low pressure would allow more flexibility in mix, a lighter, cheaper spring, but slower venting.
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Unread postAuthor: jrrdw » Thu Jul 15, 2010 4:51 pm

Back on topic so CS and I don't start kicking ass and taking names...

Micro switches, timed circuits (programmable), solenoids, bellcranks, bushings and mounting brackets are of plenty under the play field of a pinball machine. Instructions, pics and repair information can be found here and here.

Do Ebay searches if you interested in getting good deals on these type parts.

As for cooling your chamber, Igloo coolers(sp) with prefroozen ice packs could do the trick. A hole saw and some epoxy magic could do the trick...

Just some thoughts after catching back up with your project. :D
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Unread postAuthor: BigBang J » Thu Jul 15, 2010 10:36 pm

Hey guys, pardon my ignorance, but why do you need to cool the chamber? I mean sure you want your chamber to stay at a reasonable temp, but are ice packs neccesary? and doesn't the combustion work a little better at warmer temps?
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Unread postAuthor: Hotwired » Thu Jul 15, 2010 11:04 pm

BigBang J wrote:Hey guys, pardon my ignorance, but why do you need to cool the chamber? I mean sure you want your chamber to stay at a reasonable temp, but are ice packs neccesary? and doesn't the combustion work a little better at warmer temps?
The cooler the gas the better as you can get a higher density of gas in the chamber (whatever compression you're using) before igniting it.

As for cooling the chamber, depends how much heat is building up while firing.

If it can build up heat faster than it can lose it to the air at the rate it is being fired that may or may not become a problem.

Plastic or rubber parts on the metal, paint, physical contact with the cannon, expansion in mechanisms causing jams, some things get unhappy next to hot metal.

At an extreme end the gas you put in might auto ignite due to heat build up in the cannon.

Don't think I've seen any cannons round here that needed an active cooling system but there's lots of ways to increase the heat loss if you needed to.
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